FIG. 1 is a block diagram of a conventional direct drive transmitter utilized in unshielded twisted pair (UTP) applications. Referring to FIG. 1, there is shown an integrated waveform generator/encoder block 102, a timing and mode control logic block 108, a digital-to-analog converter (DAC) block 110 and a low pass filter (LPF) block 112. The low pass filter block 112 is an optional processing block that may include suitable low pass filtering and line driver circuitry. The waveform generator/encoder block 102 may include a waveform generator block 104 and an encoder block 106.
The waveform generator block 104 may be coupled so that it receives one or more transmitted input data signals 116. These transmitted input data signals may be digital signals. An output signal 120 of the low pass filter 112 may be transmitted to an unshielded twisted pair. In a case where the low pass filter block 112 is not present, output signal 118 of DAC block 110 may be transmitted to the unshielded twisted pair.
The waveform generator block 104 may include suitable circuitry and/or logic such as a digital filter, and may be adapted to generate waveform data from the transmitted input data signal 116. The transmitted input data signals are over-sampled or interpolated to produce a higher rate waveform data. For example, a 20 mega samples per second (Msps) transmitter may be adapted to interpolate by a factor of 16× will produce waveform data at 320 Msps.
An input signal 122 from the encoder block 106 may be coupled to an output of the waveform generator block 104. The encoder block 106 may include, for example, suitable DAC encoder circuitry that may be adapted to convert the generated waveform data into control words to be processed by the DAC block 110. Encoding functions executed by the encoder block 106 may be integrated with functions of the waveform generator 104 into a single waveform generation and encoder block. U.S. Pat. No. 6,411,647 to Chan entitled “Fully Integrated Ethernet Transmitter Architecture with Interpolating Filtering” assigned to Broadcom Corporation of Irvine, Calif., discloses an integrated waveform generator and encoder. Notwithstanding, additional timing functions and techniques may be integrated into encoder block 106 to provide a higher degree of interpolation, which may effectively provide over-sampling.
Referring again to FIG. 1, the timing and mode control logic block 108 includes suitable clock and selector circuitry that is adapted to control the waveform generator block 104 and the encoder block 106. The timing and mode control logic block 108 produces these controls from, for example, either clocks from a phase lock loop (PLL) or from another suitable timing source.
An input of the DAC block 110 is coupled to an output of the encoder block 106. This input of the DAC block 110 receives the output signal generated by the encoder block 106. The DAC block 110 may include suitable DAC circuitry that may be adapted to convert code words generated by the encoder block 106 into suitable waveform contained in output signal 118. The encoded code words generated by the encoder block 106 are applied to the DAC block 110 at the sampling rate such that the DAC block 110 produces waveforms at the sampling rate.
The optional low pass filter block 112 may be added to the output of the DAC block to further assist in reducing unwanted emissions. The term direct drive means that the DAC block of the transmitter 102 is sufficiently designed so that it can directly drive an output load without a need for a subsequent power amplifier. In this regard, it is not necessary to add a power amplifier to amplify the output signal 118 of the DAC block. A direct drive transmitter directly generates a transmitted waveform, namely output signal 118, on a system load. Since the timing and mode control logic block 108 of transmitter 102 is configured to control an amplitude and various timing parameters of signal 118, the timing and mode control logic block 108 alone determines the characteristics of the transmitter 102. Moreover, waveforms may be digitally produced and converted to analog signals by the DAC block 110 so that it can fit within a transmit template of various communication standards and protocols. An over-sampled direct drive transmitter such as transmitter 102 may be adapted to fulfill a variety of template requirements. Accordingly, the direct drive transmitter 102 may be suitably adjusted to accommodate various nuances of a particular template under a variety of test load conditions.
Waveforms generated by the DAC block 110 are also digitally programmable. This permits flexibility in implementing a variety of waveforms for a variety of operational modes and applications. For example, DAC block 110 may be programmed to support 10 Base-T, 100 Base-TX and 1000 Base-T applications. Since the direct drive transmitter 102 is digital programmable, it may be programmed to provide direct control of output voltage of output signal 118. This makes the direct drive transmitter 102 well suited for applications having tight absolute output voltage specifications. For example, the direct drive transmitter 102 suitable for 100 Base-TX applications. Since the transmitter 102 directly drives the load, it does not require a power amplifier which would provide additional variations in the output voltage. In certain applications, the addition of a power amplifier may add unwanted complexity to transmitter design and may require additional circuitry and/or logic to mitigate unwanted effects. Accordingly, the transmitter may require recalibration to operate efficiently. In certain instances, dependent on the transmitter design, an additional complex output filter may also be required to mitigate unwanted signal artifacts.
A direct drive transmitter such as transmitter 102 is more power efficient than other non-direct transmitters because it does not require an additional driver block to drive the line. The DAC block 110 directly takes a bias current from a high accuracy reference source, which may be provided by the timing and mode control logic block 108, and use a simple single current mirror to produce the output drive current. Other transmitter architectures which utilize a combined DAC and line driver have an additional higher power overhead.
The architecture of the direct drive transmitter such as transmitter 102 does provide some scalability and modularity. In this regard, a minimum number of analog functions are provided for optimization and this allows the transmitter design to be easily ported and adapted to new processes and applications. Some of these functions include a current mirror and differential pair. The architecture of the direct drive transmitter provides greater manufacturability and testability over other non-direct drive transmitters. The direct drive transmitter relies on an over-sampled DAC to produce the nuances of the transmit waveform instead of an analog line driver with a filter. For this reason, the clock rate can be lowered to easily evaluate the transmitter's performance and wave-shaping properties.
A direct drive transmitter such as transmitter 102, which utilizes the programmable DAC, avoids any dependence on analog filters whose characteristics can vary widely with factors such as process, temperature, and voltage variations. Hence, while analog filters may require calibration schemes which may be complex or unable to cover all varying operational conditions, the direct drive transmitter 102 is readily programmable to cover a wide range of operational conditions. Finally, the over-sampled DAC approach further ensures predictable and repeatable performance, which may be necessary in order to meet various alternating current (AC) waveform specifications.
The output of a direct drive transmitter is a differential discrete time waveform. The power spectral density of the waveform contains images which are centered around multiples of the sampling rate. For example, a 16× over-sampled 20 MHz direct drive transmitter produces images around 320 MHz, 640 MHz, 960 MHz and so on. Any sharp edges of the differential waveform may be converted to common-mode energy by effects such as mismatches in the transmitter, terminations, board traces, magnetic and transmitted medium. This conversion is called differential-to-common-mode conversion. Differential-to-common-mode conversion is more noticeable at higher frequencies because of parasitic effects and non-idealities, which cause mismatches that are more difficult to control.
Common-mode energy typically results in the emission of radiation. In general, a transmitter with high radiation emissions produces a high amount of common-mode energy. The high frequency images in a discrete time waveform of a direct drive transmitter can be readily converted to common-mode energy, which often results in unwanted radiation emissions. In such cases, the application of a simple capacitive low pass filter with a low corner frequency of, for example, less than 100 MHz, to the transmitter output is often ineffective since this will degrade return loss performance. The lowest corner frequency of a single capacitive low pass filter that will provide an optimal return loss performance is about 1 GHz. However, the use of such a filter is insufficient to reduce the high frequency images produced by the direct drive transmitter. This is particularly true in network systems that have high port densities. For example, a 48-port Gigabit switch that utilizes a direct drive transmitter having a single capacitive low pass filter, will have extreme difficulties adhering to emission radiation regulation specifications. This is because the aggregation of emission radiation from the high number of transmitters utilized will readily exceed emission radiation limits established by the regulation specifications.
One solution geared at reducing the aggregation of the emissions from the transmitters includes increasing the over-sampling rate of the direct drive transmitter and increasing the order of the digital filtering. However, this solution will increase the complexity of the DSP filter and clocking speed by a factor equivalent to the increased over-sampling rate. For example, doubling the over-sampling rate will double the hardware and complexity of the transmitter Accordingly, such as method would not be desirable.
FIG. 2 is a graph 200 illustrating a differential waveform power spectral density (PSD) for the direct drive transmitter of FIG. 1. Referring to FIG. 2, the vertical axis represents power and the horizontal axis represents the frequency. A baseband signal 202 is centered on a frequency of zero (0). A plurality of image frequencies are centered on frequencies that are multiples of the sample frequency (Fs). For example, a first image frequency 204 is centered on a frequency of Fs. A second first image frequency 206 is centered on a frequency of 2*Fs. A third image frequency 208 is centered on a frequency of 3*Fs. A fourth image frequency 210 is centered on a frequency of 4*Fs and so on.
The power (P) of the image frequencies are represented as follows:
 P=Power of Baseband Signal*[sin(2*π*f/Fs)/(2*π*f/Fs)]^2,
where f represents the frequency and Fs represents the sample frequency.
As the frequency increases, the power of the image frequencies decreases. Differential to common mode conversion of the image frequencies causes emission radiation. The use of a simple low pass filter is effective in filtering only the higher image frequencies. For example, the use of a simple low pass filter may be effective in filtering the images at 2*Fs and greater. Notwithstanding, the use of a simple low pass filter will not be effective in filtering the first image frequency at Fs since it is the largest of the image frequencies. Although increasing the over-sampling rate will reduce the image frequency at Fs, over-sampling rate will double the transmitter hardware, thereby increasing transmitter cost and complexity.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.